1. Field of the Invention
The present invention relates to an apparatus for measuring bio-magnetic fields by using SQUID (Superconducting Quantum Interference Device) magnetometers or the like to detect weak magnetic fields generated from the heart or brain of an adult, an infant, or a fetus. More particularly, the present invention relates to an apparatus for measuring the bio-magnetic fields detects in real time changes of the magnetic fields of the live body when a high-frequency AC current is applied to the body and for calculating and displaying the changes in a two-dimensional map.
2. Description of the Related Arts
The conventional ways for measuring bio-magnetic fields include detecting the changes of magnetic fields caused by neuron actions in brain cells, which provides magneto-encephalograms, or by electric current activated in cardiac muscle cells, which provides magneto-cardiograms.
A new method named xe2x80x9cimpedance cardiographxe2x80x9d is designed to measure the electric potential changes, that appears as the amount of blood flowing in a live animal body or a human body of an adult, an infant, or a fetus, when the body is subjected to a high-frequency AC current. (Aerospace Medicine, vol. 37, pp. 1208-1212 (1966), Aviation, Space, and Environmental Medicine, vol. 70, No. 8, 780-789 (1999).
The Japanese Patent Laid-open No. 225860/1994 discloses an apparatus for measuring the spatial distribution of electric impedance. This apparatus supplies electric current to a to-be-examined region in the live body through at least two electrodes and electric wires so as to trigger impedance and current which distribute corresponding to the positions of the electrodes, thereby detect the spatial distribution of the characteristic quantities of the magnetic field generated by the current distribution at measuring points outside the to-be-examined region by means of a magnetic field measuring apparatus, and reconstructs the equivalent current density distribution inside the to-be-examined region from the spatial distribution of the characteristic quantities. The apparatus is thus highly sensitive to the magnetic fields generated by electric wires in the to-be-examined region by using a compensation conductive loop.
The above-mentioned apparatus regarding magneto-encephalograms and magneto-cardiograms share one disadvantage that they merely monitor the electric phenomena in cells but not the two-dimensional mechanical movement of the heart magnetic fields. Another disadvantage of the conventional impedance cardiograph based on potential measurements is the use of a large number of electrodes for monitoring the blood flowing in the region, which is undesirable for general use, such as clinical diagnostic.
For these reasons, there has been a demand for a technique for monitoring in real time and in a non-contacting manner the magnetic fields which change along with the mechanical movements of blood flow, cardiac contraction and expansion, or the like. There has also been a demand for a technique for measuring in real time the mechanical movement of blood simultaneously with the electrical activity of cardiac muscle and brain thereby drawing a two-dimensional map.
The Japanese Patent Laid-open No. 225860/1994 discloses a technique for detecting the distribution of electric impedance at a certain time created by the electric current flowing in the live body through the electrodes, but it does not detect in real time the change of electrical impedance.
It is an object of the present invention to provide an apparatus for measuring bio-magnetic fields of a live body. The apparatus is designed to measure the magnetic field from the heart or the magnetic field from the brain which is generated by the electrical activity of muscles and nerve cells in the live body and to detect simultaneously the mechanical movement induced by the blood flow in organs or the movement of organs in the live body,xe2x80x94so as to monitor them in real time in a two-dimensional manner.
In order to more clearly and concisely describe the subject matter of the claims, the following terms are intended to provide guidance as to the meanings of specific terms used in the following written description. Also it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. As used herein:
xe2x80x9cCardiac magnetic fieldxe2x80x9d denotes the magnetic field generated by the electric activity of cardiac muscles. This magnetic field can be detected by a SQUID magnetometer or the like.
xe2x80x9cMagneto-cardiogram waveform (MCG)xe2x80x9d denotes the waveform of a cardiac magnetic field.
xe2x80x9cMagneto-cardiogram (MCG map)xe2x80x9d denotes a two-dimensional contour map of a cardiac magnetic field (an magnetic field distribution map of contour lines formed by connecting points having an equal magnetic field magnitude).
xe2x80x9cImpedance cardiac magnetic fieldxe2x80x9d denotes a magnetic field generated from the high-frequency AC current applied to a live body. This magnetic field is detected by a SQUID magnetometer or the like.
xe2x80x9cImpedance magneto-cardiogram waveform (I-MCG)xe2x80x9d denotes the waveform of an impedance cardiac magnetic field.
xe2x80x9cImpedance magneto-cardiogram (I-MCG map)xe2x80x9d denotes a two-dimensional contour map of the impedance cardiac magnetic field (an magnetic field distribution map of contour lines formed by connecting points having an equal magnetic field magnitude).
According to one embodiment of the present invention, the apparatus for measuring bio-magnetic fields comprises a plurality of SQUID magnetometers (which are arranged in a cryostat maintained in a superconducting state) for detecting the magnetic fields generated from the live body, and driving circuits for driving the SQUID magnetometers. The apparatus is connected to electrodes which are attached to more than one point of the subject under examination, such as the head, leg, etc. The high-frequency AC current generated from a signal generator, such as an oscillator, is supplied to the electrodes. The output signals of the driving circuits are processed by two signal processing circuits.
The first signal processing circuit includes a band-pass filter, a notch filter, and an amplifier. The second signal processing circuit includes a high-pass filter, a phase-shift detector, a band-pass filter, and an amplifier.
The output signal (I-MCG) from the second signal processing circuit is displayed on a display unit in correspondence with each position where a detecting coil of the SQUID magnetometer is arranged.
The magneto-cardiogram (NCG map) and the impedance magneto-cardiogram (I-MCG map) are obtained by processing with a computer based upon the magneto-cardiogram waveform (MCG) and the impedance magneto-cardiogram waveform (I-MCG) at an arbitrary time.
The output signal (MCG) from the first signal processing circuit and/or the output signal (I-MCG) from the second signal processing circuit may be displayed on a display unit in correspondence with each position where a detecting coil of the SQUID magnetometer is arranged.
More than one I-MCG may be displayed (ex. in an overlapping manner) on a display unit. Likewise, more than one MCG may be displayed on a display unit.
The impedance magneto-cardiogram (I-MCG map) obtained from the output signal (I-MCG) of the second signal processing circuit may be displayed on a display unit simultaneously with the MCG map obtained from the output signal (MCG) of the first signal processing circuit.
By using signals triggered by a waveform of the electrocardiogram (obtained by an electrocardiograph) which is measured simultaneously with the magneto-cardiogram waveform (MCG) and the impedance magneto-cardiogram waveform (I-MCG) or the R-wave of the magneto-cardiogram waveform (MCG), the computer performs an averaging process for the impedance magneto-cardiogram waveform (I-MCG), and an arithmetic processing of first-order differentiation on the impedance magneto-cardiogram waveform (I-MCG) with respect to time so as to provide at least one waveform of the magnetic fields due to first-order differential. The computer further processes the waveform to provide a contour map (a chart showing the distribution of equal magnetic field magnitudes) in which points of equal first-order differential values at an arbitrary time are connected. The resulted map is displayed on a display unit.
Moreover, the computer processes the waveform of first-order differential of magnetic field magnitudes to reconstruct the distribution of electric current or the distribution of conductivity (or resistivity) in the live body at an arbitrary time. The distribution thus obtained is displayed as a two-dimensional or three-dimensional pattern on a display unit.